Thieves Bane

Warlock Invocation
Level: Lesser, 3rd lv.
Components: V, S
Casting Time: 1 standard action
Range: 100 feet+10/lv
Duration: Permanent, till triggered
Saving Throw: Yes, reflex for half
Spell Resistance: No

So named for the people who most commonly suffer its effects, this invocations allows you to make a deadly trap of an ordinary door. You can produce a hold portal effect, as the spell. In addition, should anyone other then you open the ensorceled portal from the outside by any means (Including knock or dispel magic spells) before the spell's duration expires, the portal (or a large portion thereof) instant;y explodes outwards, dealing 5d6 points of damage to all within 20 feet of the door on that side. Creatures standing on the inside of the door are unaffected. Each creature but the one that triggered the effect (assuming it was standing in front of the door) can attempt a reflex save for half damage. Spell resistance does not apply to damage caused by this effect.

(Cityscape, Page 68)